Grammatically correct, yet improper?

Question

Yesterday, I mentioned the fact that I was grateful for having someone play music for me, and letting me play music as well. I first said something like:

ピアノを弾いてもらったり、弾かせてもらったりして、嬉しいです。

Then, I thought that since I was using twice "もらったり," I should be able to factorise it and express the same opinion:

ピアノを弾いたり、弾かせたりしてもらって、嬉しいです。

Then, I was told that the sentence was now incorrect. It seems to me that this is perfectly correct, as far as grammar goes. So, is it correct/acceptable, or is it again one of those "no one says it, so it's not correct" sentences?

Answer 1

You are aiming the structure

ピアノを[弾いたり、弾かせたり]してもらって、嬉しいです。

Since the predicate outside of factorization is もらう, the surface subject of it is 私 rather than あなた.

(私は)ピアノを[弾いたり、弾かせたり]してもらって

Now, consider the role of 私 in the two verb phrases that are conjoined.

(あなたが)(私に)弾いたり

(あなたが)[(私に)弾]かせたり

In the first conjunct, the role of 私 is the benefactee. In the second, it is the embedded (dative) subject of a causative construction. Since these phrases are also the surface subject of the common predicate もらう, they have to move out (factorize, in your terminology). However, it is known that language does not allow factorization of an item from different structural positions in conjoined elements. Technically, this is known as Across the Board Generalization exception to The Coordinate Structure Constraint. That is the reason it will not work.

Now, let us consider what you can do instead. The problem arose from trying to move 私. Instead of doing movement, you can leave them implicit and move the common subject of the two conjuncts: あなた. Then, あなた will occupy the surface subject position of the predicate outside, so the predicate will now have to be the one that fits in meaning with having あなた as the subject. That is くれる. Hence you get the grammatical sentence:

ピアノを弾いたり、弾かせたりしてくれて、嬉しいです。